1. Field of the Invention
The present invention is a process whereby yield, raffinate product quality, and throughput of the selective aromatics solvent extraction of hydrocarbons are improved by subjecting the hydrocarbon feed prior to the selective solvent extraction step to a membrane separation process whereby aromatic hydrocarbons are selectively removed from the feed by permeation through a membrane. Said selective membrane permeation step produces a permeate enriched in aromatics and a retentate of reduced aromatics content. Practice of the selective aromatics solvent extraction process on the retentate results in a higher yield of raffinate of higher quality and higher throughput as compared to the practice of the selective aromatics solvent extraction process on the raw hydrocarbon feed under the same extraction conditions (i.e., extraction severity).
The membrane separation process practiced can include any of the procedures for separating aromatic hydrocarbons from feed streams containing mixtures of aromatic and non-aromatic hydrocarbons. Such procedures include pervaporation, perstraction and membrane extraction processes.
2. Description of the Related Art
Removal of aromatic hydrocarbons from hydrocarbon feed streams such as fuels or lubes or specialty products (e.g. refrigerator, turbine, electrical insulating or white oils) is a commonly practiced process. This is so because the presence of aromatics in such hydrocarbon products is usually detrimental to their performance and commercial unacceptability.
Aromatic hydrocarbons in lube oil fractions have been associated with reduced viscosity indexes and poor stability to oxidation and light. For this reason it is generally beneficial to remove the aromatics.
Despite this generally accurate statement, however, it is equally true that not all aromatics are undesirable lube components or detrimental to performance or quality.
Poor oxidation and light stability is now associated with the polynuclear aromatic compounds, i.e. the multi-ring aromatics. Indeed, the presence of 1-ring aromatics in a lube oil fraction may be beneficial with regards to viscosity index and oxidation and light stability. One-ring aromatics which are heavily branched with alkyl side chains are now viewed as being desirable lube oil constituents.
The typical way to remove aromatic hydrocarbons from hydrocarbon feeds is by solvent extraction. In such a process the hydrocarbon feed is introduced into an extraction zone and contacted with a selective aromatic extraction solvent moving countercurrently. Typical aromatics extraction solvents include phenol, furfural, sulfolane, and N-methyl 2-pyrrolidone (NMP).
This process produces a raffinate rich in saturates and an extract lean in saturates and rich in aromatics present in the extraction solvent.
The raffinate is recovered for use as a lubes base stock while the extract, following solvent recovery to yield an extract oil may be further processed for specialty products or is used as cat cracker feed, burnt as fuel or sent to a coker.
U.S. Pat. No. 4,802,987 teaches the selective removal of aromatic hydrocarbons from feed streams containing a mixture of aromatic and aliphatic hydrocarbons by selectively permeating the aromatics in the feed under pervaporation conditions through a regenerated cellulose or cellulose acetate membrane having a dry thickness of about 10 to 25 .mu. and a molecular weight cut off of from about 10,000 to 50,000 which membrane is impregnated with from 10 to 25 wt% polyethylene glycol having a molecular weight in the range of about 600-14,000. In the text at column 1, line 35 the patent suggests that the process "can selectively remove aromatics from these mixed feed streams to reduce the severity of solvent extraction or eliminate distillation." This can be taken to mean that should aromatics be taken from a hydrocarbon feed stock by a prior membrane step then it would be possible to "back-off" on the extraction process and operate the latter at reduced severity (e.g. lower treat rate and/or lower temperature) to achieve the same raffinate product quality/quantity. It would be presumed that if some of the aromatics are first removed by membranes then less would have to be removed by the subsequent extraction step and such step could be run at reduced severity. Nothing in the reference suggests that solvent extracting a retentate from the membrane separation of a distillate under standard extraction conditions would result in an increased yield of higher quality raffinate as compared to simply the extraction, under similar conditions, of the same distillate. At best, it can be speculated that extracting the retentate at lower severity as suggested in U.S. Pat. No. 4,802,987 might achieve a yield advantage but this might have been accompanied by some raffinate quality decrease. That both quantity and quality of the raffinate obtained is improved in an unexpected synergy of the 2-step process of the present invention.